Add the ability to use assumed roles in S3A
- Add a property fs.s3a.assumed.role.arn for the ARN of the assumed role
- add a new provider which grabs that and other properties and then creates a STSAssumeRoleSessionCredentialsProvider from it.
- This also needs to support building up its own list of aws credential providers, from a different property; make the changes to S3AUtils for that
- and have the AwsProviderList forward closeable to it.
- Get picked up automatically by DDB/s3guard